Epoxies are an important class of thermosetting polymers. They have diverse applications including adhesives, structural materials, paints coatings, potting, printed circuit boards, microelectronic encapsulation, the aerospace industry, and other consumer goods. Epoxy resins are hardened or cured by a cross-linking reaction using one of three methods. The chemistry of epoxy curing is explained in great detail in the Handbook of Composites (edited by S. T. Peters, Chapter 3, pp 48-74, published by Chapman & Hall, 1998). The properties and applications of cured resin are greatly influenced by the choice of the hardener formulation or the method of curing.
One method is simply the reaction of the epoxy resin with itself (i.e. homopolymerization) via a ring-opening polymerization mechanism of the epoxy groups. The self-curing of epoxy resins usually requires an elevated temperature but can be initiated with either a Lewis acid or a Lewis base catalyst (as opposed to a curing agent).
In the second method, the epoxy resin is cured with a cyclic acid anhydride. The anhydride can react with the epoxy group, pendant hydroxyls, or residual water to form a carboxylate intermediate, which then reacts with the epoxy group, causing a self-perpetuating reaction between the anhydride and the epoxy resin. Catalytic amounts of tertiary amines are commonly used as additives as they facilitate the opening of the anhydride. Anhydride epoxy formulations do not readily cure at room temperature, and generally require a significant room temperature of 80-150° C.
In the third method, the epoxy resin reacts in the ambient with polyvalent nucleophiles such as polyamines to form a polymeric network of essentially infinite molecular weight. Polyamines of the general formula (NH2—R—NH2) give cold curing compositions. The ring opening of the epoxy ring with a primary or secondary amine generates a stable C—N bond. Epoxy groups will react with potentially every amine containing an active hydrogen atom, so that a simple diamine (NH2—R—NH2) acts as a tetrafucntional cross-linker and reacts with four epoxy groups. Similar to amines, polythiol compounds (HS—R—SH) also react with epoxy rings to form C—S bonds. The reaction of the thiol group with the epoxy group is greatly facilitated by the presence of a catalytic amount of base, such as a tertiary amine. A simple dithiol compound (HS—R—SH) serves only as difucntional chain extender since a primary thiol contains only one active hydrogen atom, but polythiol compounds with a functionality greater than three serve as cross-linkers. Polythiol hardeners also allow for ambient curing compositions. Faster setting formulations, which are commonly sold as two-pack glues in hardware stores, usually contain polythiol hardeners or both polythiol and polyamine hardeners.
By far, the most common epoxy formulations consist of a diepoxide (“resin”) and a polyamine (“hardener”) to form a polymeric network of essentially infinite molecular weight. The combination of “resin and hardener” is sometimes referred to as “cured epoxy,” “cured resin,” or simply “resin” or “epoxy.” The widespread utility of such epoxy formulations is due to their excellent processability prior to curing and their excellent post-cure adhesion, mechanical strength, thermal profile, electronic properties, chemical resistance, etc. Furthermore, the high-density, infusible three-dimensional network of epoxies makes it an extremely robust material, resulting in it being the material of choice for many long-term applications. At the same time, this durability makes its removal, recycling and reworkability notoriously difficult, raising concerns about the longevity of epoxy-based materials in the environment. The cross-linking reactions that occur with two convertibly used component epoxies are essentially irreversible. Therefore, the material cannot be melted and reshaped without decomposition of the material. The ordinary consumer is also aware of the intractability of epoxy adhesives and coatings; internet message boards are replete with postings and complaints on how to remove epoxy that has spilled on unwanted places or mistakenly bonded items together. Thus, there exists a need for new epoxy formulations that retain the remarkable physical properties of classical epoxies, but can be disassembled in a controlled and mild manner when desired, without damaging the underlying structure.
As epoxy adhesives are used for the assembly of a variety of common items and epoxies serve as the matrix materials for a variety of structural materials and composites, the development of such a “reworkable” material would have implications in recycling, recovery, and waste disposal. Furthermore, an easily removable epoxy could expand the use of epoxies to new consumer markets. For example, joints could be bonded with epoxy glue and any spill-over could be easily removed, even post-curing, while the joint remains bonded. As another example, epoxy based paints and varnishes could be more easily removed.
The intractability of a cured resin stems, in part, from its highly cross-linked network. If the links in the three-dimensional network can be cleaved under controlled conditions, the network can be disassembled into smaller, soluble molecules and/or polymer, therefore removing the cured resin stem. In principal, this can be accomplished through use of either a dissolvable resin or a curing agent that contains a bond capable of cleavage under a specific set of conditions. In the limited amount of prior art on this topic, the majority has focused on cleavable groups in the resin component. Epoxy formulations that possess cleavable linkages in the hardener, are particularly attractive, as those skilled in the art realize that a great deal of more flexibility exists with regard to the constituents in a hardener component, due to the resin components in most epoxies are based on bisphenol digylcidly ether (BPADGE).
U.S. Pat. No. 5,932,682 discloses the use of diepoxide resins that contain ketal or acetal linkages for use as removable electronic encapsulation. The anhydride cured resins were shown to disassemble in acid at an elevated temperature. There are no examples of curing the resins with polyamine, polythiols, or acid liable hardeners. Further, the use of acid-sensitive linkages in the hardener component, or both hardener and resin, has not been previously documented or considered to those skilled in the art.